The receiver features an audio amplifier with low internal noise. This makes it useful for tape recording weak signals.

A small VLF antenna can be made that works with this unit. See the section on VLF ANTENNAS below on this page for antenna construction.

By successfully building this VLF receiver you will learn how basic electronic circuits work and how audio amplifiers work. We've kept it simple using only transistors and individual parts to allow you to learn how basic electronic parts work. The design is simple yet delivers loud.

There is no volume control shown, but you can add one if you want to. Most VLF receivers of this type are operated at full gain.

The VLF receiver can be used for radio astronomy in the detection of solar flares and other events in space, and for monitoring lightning activity. Computer software is available for monitoring and data logging. The receiver provides enough power output to drive our serial port interface.

When used for radio astronomy, you can monitor natural static levels which will decrease or increase suddenly if solar flares occur. It is not the actual lightning flashes that are changing, rather it is the ionosphere that is affected causing a sudden loss of long distance signal reception, or a sudden increase in long distance signal reception.

When used as a lightning detector you can monitor lightning activity with your computer. It will drive our serial port interface and Version 3 Lightning Detection Software.

Construction Details:

There is no technical help with the projects. This circuit is shown as an educational example only.

Printing the circuit diagram: right click and select "print". Or you can use "Photo Impression" software.

Printing tips: Some photo programs allow you to set the print height and width: The image width should be set at 7.3 inches. The image height should be set at 3.8 inches. The resolution should be around 96 dpi.

Do NOT darken the circuit diagram lines. The image will print looking gray. This is required for correct operation. The ink used in most computer printers is electrically conductive and will adversely affect circuit operation if you were to darken the ink lines. The image prints in gray so that microscopic dots form, keeping the printed image from being electrically part of the circuit.

The image contains a ruler reference that should be 7 inches long when printed to exact size.

You want your radio to look good so if you take your time, it may take up to 6 days working an hour at a time to finish the receiver completely.

Enjoy the building part of it no matter how long it takes you!

To build the receiver circuit on the copper tacks or copper weather strip nails (see photo below), a 15 watt soldering iron is what you need, unless using wire-wrap methods.

There is no technical help with the projects. This circuit is shown as an educational example only.

If you use copper plated weather strip nails, the size of the nails is called "3/4 X 17".

IF YOU CAN'T SOLDER, OR NEVER HAVE, (and don't want to learn how today...), Radio Shack stores have "perforated circuit board" and wire-wrap parts. You can build the radio without needing to solder anything. We built the VLF radio firstly on a solderless breadboard (just to fool around with it before permanently wiring it) and it worked fine even with the stray capacitance of the metal base. It did not squeal or oscillate even when built in such a "poor way" just for testing.

If soldering, use eye protection goggles (solder or the solder's hot wet flux can splatter). Open a window and use fan ventilation - solder fumes are poison says the fine print. Read all warnings.

Print out the above circuit diagram template. It should print out to 7 inches wide. A built in scale is shown for accuracy if you need to scale it for size on your printer.

Construction is done by using simple materials.

The receiver circuit itself can easily be constructed on a piece of dry wood. The dots show where to hammer in the copper tacks or copper weather strip nails.

For our receiver we placed the printed paper circuit card onto a small wood board and hammered in all the copper weather strip nails.

Just hammer a copper tack or nail (available from any local hardware store) in each dot in the diagram and solder the parts together.

There is no technical help with the projects. This circuit is shown as an educational example only.

Mounting the Resistors:

Place all the resistors first. You can wrap the resistor's wires around the copper nails or you can cut the leads and place them on top of the nail heads like shown in the photo above. This takes more time and care, but if you ever need to make changes or repairs you can easily remove the parts without damage.

TIP: When laying the parts on top of the nails, use a piece of electrical tape placed on one end of the part to hold it down until you get the other end soldered.

All resistors are rated at 0.25 watt (1/4th watt), except for R15 which is rated at 1/2-watt.

Resistor R1 is 470 ohms. Start with this one. Solder it. Move on to the next one.

There is no technical help with the projects. This circuit is shown as an educational example only.

Place all the capacitors secondly. Some capacitors have (-) and (+) markings. Install these correctly else the receiver will not work.

You can wrap the capacitor's wires around the copper nails or you can cut the leads and place them on top of the nail heads like shown in the photo above. This takes more time and care, but if you need to make changes you can easily remove the parts without damage.

Capacitor C1: 0.001 uF.

Capacitor C2 is 0.22 uF.

Capacitor C3 is 0.22 uF.

Capacitor C4 is 220 uF.

Capacitor C5 is 1000 uF.

Step 3: Place all the other wiring.

Connect the jumper wires: Use no 22 copper wire and wrap it around the copper nails and then solder it. Don't forget any connections. The photo above shows how to do it with bare copper wire (bottom ground line) and the top positive line (green) with insulated wire.

Step 4: Tin the tops of the copper nails where the transistors will sit, if not already done.

Mounting the Transistors:

There is no technical help with the projects. This circuit is shown as an educational example only.

There is no technical help with the projects. This circuit is shown as an educational example only.

Choosing a speaker for your receiver: We recommend any speaker of 3 inches to 8 inches. Your speaker should reproduce sounds in the 300 Hz to 10000 Hz range well. Any "fullrange" speaker is recommended.

Any 4-ohm, 8-ohm, or 16-ohm speaker. Frequency response should be in the range of 300 Hz - 10000 Hz.

DO NOT USE HEADPHONES OR EARPHONES! LOUD SOUND!!!!!!!!!!!!!!!!

There is no technical help with the projects. This circuit is shown as an educational example only.

The VLF receiver circuit can be built into a into a wooden box. Mount the antenna at least 12 inches away from the speaker to prevent feedback squeeling!!

How the front panel looks. The input is on the left. The audio output is on the right below the speaker.

We chose binding posts for input and output. The speaker will not turn off when the external output is used. This is useful for monitoring signals while recording, etc.

How the back of the front panel looks. The decision was made to add a signal strength meter to the front panel. The receiver will work with or without the meter but it would be good to have a nice meter.....But it can be a challange (wood-working-wise) to add it so it looks good.

The completed receiver, except for top and back wood cover. We added a signal strength meter. See meter wiring diagram below.

There is no technical help with the projects. This circuit is shown as an educational example only.

The completed receiver (with top and back wooden panels removed) with added meter.

The meter helps in finding the lowest noise null point when aiming your antenna.

Adding the meter:

We added a signal strength meter. The meter will deflect for each static crash or tweek received. Weak signals give low readings. Strong signals give high readings. The meter can not slam off scale because of the current limit-setting resistor.

The meter circuit:

The Parts for the Signal Meter Circuit:

30 uA Analog Panel Meter: MCM Electronics Part # PM11271.

Resistor: R1 1 Meg ohm.

Resistor: R2 470 K-ohms.

Resistor: R3 150 K-ohms.

Capacitor: C1 0.022 uF

Capacitor: C2 0.1 uF

Capacitor: C3 1 uF

Diode: D1 1N4148 or 1N914

Diode: D2 1N4148 or 1N914

Transistor Q1: 2N4124

Transistor Q2: 2N4124

Place transistors lastly to prevent static or heat damage.

Build this small meter driver circuit on a small piece of wood board. Use copper nails and solder the parts like before.

Connect the power leads to the battery supply line of the receiver.

Connect the meter to the meter output.

Connect the input lead to the speaker.

Need more gain? See project 2 for a FET pre-amp that can be connected ahead of this unit's input. You can even combine the two onto one project!

There is no technical help with the projects. This circuit is shown as an educational example only.

VLF Antennas

Any of our "N" or "A" or "B" line of VLF antennas will work with the unit.

The unit can be modified to go down to ELF frequencies so you can use it with our ELF antennas. Use bigger value capacitors in the inputs and outputs to allow the lower Hz ranges to pass.

POWERING UP:

When the antenna is connected and mounted more than 12 inches away from the speaker, you are ready to turn the power "ON".

Turn the power on and after a few seconds sounds should begin from the speaker. The C6 capacitor in the circuit needs a few seconds to charge up when first turned "ON".

If meter stays full scale then the unit is likely receiving an ultrasonic (above 20 KHz) VLF signal. Try placing a capacitor across the input to adsorb the high frequency, or increase the value of C1.

Specifications: Current draw when idle is about 24 milliamps at 9 volts.

When powered up, the receiver should draw approximately the current level listed above.

WARNING!: THIS RECEIVER DOES NOT HAVE A VOLUME CONTROL SO DO NOT LISTEN THROUGH HEADPHONES OR AN EARPHONE. SUDDEN VERY LOUD SOUND IS POSSIBLE! USE SPEAKER ONLY!
Speaker Grille Templates you can print and use.

Here are 3 designs for speaker grilles. We drilled through the front panel wood and mounted the speaker behind the grill. Takes a lot of time to drill the holes but its better than cutting out a large circular hole to mount the speaker.

You may choose to use your own design you make on a paint program on your computer. Make sure the grille is big enough for your speaker and that you have a minimum of 100 holes for the sound to go through. The red one above has 116 holes. Drill where the lines cross.

Make sure to put a back cover on your receiver and drill a few holes in the cover so the sound can come out the back. This makes the radio's volume even louder and adds "richer" tones. You may want to experiment with different back covers and sound port-holes or speaker mounting or sound wave-guides to get the exact "custom" sound you want.

TROUBLES?

We had no trouble building the radio. The radio worked from the start without any squeeling, hissing, or other electrical problems.

Start with a brand new 9-volt battery. As battery ages the volume will become lower so start out with a fresh battery!

You can check the various stages of the receiver as follows:

Touching R1 will produce a loud buzzing sound from stray hum noise.

The audio amplifier requires use of the resistor and capacitor values listed, else it will not work correctly.

There are 4 NPN transistors used in the radio. Make sure you carefully place the "E", "B", and "C" leads of the transistors to the correct tack points of the circuit diagram.

Poor sensitivity if all else checks out? Is the ferrite rod or antenna too close to the speaker's magnet? Mount speaker and ferrite rod at least 12 inches apart.